Venus glows overhead

November 12, 2013

The Adirondack nights are again lengthening as Earth's orbital motion tilts the northern hemisphere away from the sun. As we rise and leave for work in the mornings and return in the evenings, our eyes and minds are drawn to the vast wilderness above and this column returns to guide you to wonders we can see and those beyond our sight that only the most powerful instruments can detect.

To begin, turn to the southwest tonight about an hour after sunset to see bright Venus in the midst of the Teapot, an asterism in Sagittarius. At sunset, 4:34 p.m. in Tupper Lake, Venus will be 15 degrees above the horizon, about 45 degrees south of where the sun set. It will emerge from the darkening sky quickly, but the stars around it will take longer. By about 5:30, the stars of The Teapot should start becoming visible. An asterism is a recognized pattern of stars that is not one of the 88 official constellations recognized by the International Astronomical Union. The Big and Little Dippers are asterisms in the constellations of Ursa Major and Ursa Minor. The Teapot, and accompanying Teaspoon, are part of the constellation Sagittarius, The Archer, often represented as a Centaur drawing a bow. The bow is represented by Kaus (rhymes with mouse) Borealis, Kaus Media and Kaus Australis. "Kaus" derives from the Arabic word for "bow", but these are mixed names with Borealis, Media and Australis being Latin for northern, middle and southern. The tip of the arrow (aimed at Scorpius), is represented by Alnasl (All-NOZ-zl), Arabic for "The Point." Ascella (Ah-CEL-lah), in the Teapot's handle, represents "the armpit of the archer," according to the translation of its name.

Over the next 12 days, as shown by the diagram, Venus will appear to move through Sagittarius. In fact, Venus will appear each evening in about the same location as it will tonight. However, due to Earth's orbital motion 360 degrees in 365 days our nighttime window shifts about one degree eastward each day (so the stars shift west).

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Over the next week we have an excellent opportunity to actually notice this motion, thanks to Venus and the western horizon. If it's possible for you to observe at the same time for a few nights in a row, note the positions of the stars in this lovely constellation as they slip into the west to emerge again from behind the sun in the mornings of late January.

Known as both Hesperos, the evening star, and Phosphoros, the morning star, Venus never ventures farther than 46 degrees from the sun since its orbit is within the Earth's. Back on March 28, Venus was on the far side of the sun. Since then it has been circling toward Earth on its faster orbit and moving eastward, away from the sun in the sky. As it swings toward us, it reached a maximum angular distance from the sun, its greatest eastern elongation, 47.1 degrees at 4 a.m. on Nov. 1. Elongation in astronomy doesn't refer to something getting stretched, but simply a celestial object's angular distance from the sun. At sunset tonight, it's still 47 degrees from the sun, but that angle is measured along the Ecliptic, not from the horizon. As shown on the diagrams, the Ecliptic is at a low angle with respect to the western horizon at this time of year. This is why Venus is so far south of where the sun sets. As its orbital motion brings it around to pass between Earth and the sun next January, it moves closer to the sun in our sky. One would think this would take it closer to the horizon at sunset. However, at the same time, the angle between the horizon and the Ecliptic is increasing so Venus will seem to hang at about the same height above the horizon (but move north) until it rather suddenly plunges toward the sun in very late December.

People are often startled by the brightness of Venus, and when it first shows up low in the morning or evening sky, reports of UFOs and complaints about "new streetlights" increase. The reasons for its brilliance are its size (95 percent the radius of Earth), proximity (one-third of Earth's distance to the sun at its closest), and the fact that the surface is completely covered with clouds. These clouds, however, are not like Earth's clouds, bringing shade and rain or snow! Venus' clouds are made of sulfuric acid and so thick that anyone living on Venus would not know there are stars and see only murky, orangish light from the sun 30 percent closer than it is to Earth. But finding anything alive on Venus would be truly startling as carbon dioxide (CO2) makes up 96.5 percent of its thick atmosphere with a surface pressure 92 times that we enjoy on Earth, the equivalent of standing under 3,100 feet of water - nearly four times the depth of Lake Ontario! Though the sunlight is murky, it does heat the ground that then emits infrared radiation that is absorbed by the atmosphere in a runaway greenhouse effect, heating the surface to an astounding 867 degrees F. In the 1960s to 1970s, the Soviets landed 10 probes on Venus that revealed a rocky surface in the few moments they survived in its harsh environment.

They also mapped the surface with radar, a feat that was repeated at higher resolution by the American Magellan spacecraft in the early '90s. These show a surface with more than 1,600 volcanoes and few impact craters since the thick atmosphere burns up all but the largest meteors. It has been thought that the volcanoes of Venus are mostly extinct; however, recent observations by the European Space Agency's Venus Express have sshown spikes in the atmosphere's sulfur dioxide (SO2) content. Since this compound is quickly destroyed by ultraviolet sunlight, these increases may indicate active volcanoes on the surface or some unknown mechanism dredging SO2 up from below the thick cloud decks where it's protected from the sun's UV rays.

Venus, namesake of the icon of female beauty and a glittering gem lighting these late autumn evenings - it yet hides much from our instruments, but slowly we are getting to know this brightly shrouded neighbor.

Again this year, other astronomers of the Adirondack Public Observatory will be contributing to this column. So look for articles with different viewpoints and styles to enhance your enjoyment of our wonderful dark skies. Beyond this column, local astronomers are anxious to share our love of the sky with you.

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Check out the Adirondack Public Observatory website at apobservatory.org for events. Listen for me on North Country Public Radio about once a month during "The Eight O'Clock Hour," or email me with any questions at aodonoghue@stlawu.edu.